It is a requirement that a stairlift includes an OSDD to detect if the stairlift carriage exceeds a speed prescribed in regulations. If there is a possibility of drive failure leading to free-fall of the carriage, the OSDD must be combined with a safety gear mechanism which, when triggered by the OSDD, will bring the stairlift carriage to a halt.
There are two main classes of OSDD and/or OSG. The first class operates by means of one or more weights that displace, typically centrifugally under excess speed, to provide a triggering action. An example of this type of OSDD/OSG is described in our European Patent No. 1 149 041. A second class of OSDD generates an electronic signal representative of the speed of the stairlift carriage and compares this with a pre-determined threshold to decide if a triggering action, typically the triggering of a safety-gear mechanism, should be effected. An example of this type of OSDD/OSG is described in our European Patent No. 1 539 628.
Regulations currently in force governing the construction and operation of stairlifts do not accommodate OSDDs of the second class and, accordingly, the OSDD and OSG described and claimed herein belongs in the first class. Certain requirements and limitations apply to OSDDs and OSGs, particularly of the first class described. These include:    i) There is limited space in the stairlift carriage to accommodate an OSDD and OSG and thus the physical dimensions of the device should be as small as possible.    ii) Because of the geometry of the OSG described in EP 1 149 041, and of similar devices, the tripping speed varies with changes in the vertical inclination of the rail. It therefore follows that a trip speed range must be provided based on both rail angle and carriage speed. This, in turn, limits the overall speed capability of the stairlift if unintended tripping is to be avoided.    iii) Currently OSDDs and OSGs of the first class are driven at the same speed as the carriage, yet regulations require the stairlift carriage, in the event of drive failure, to be brought to a halt before the carriage has moved a significant distance, and without the chair going off-level by more than 10°. Clearly, in the event of failure, if the carriage is moving more slowly than a normal rated speed, a longer distance will be required for the OSDD and/or OSG to operate.    iv) Following on from iii), as a stairlift carriage is driven through a negative transition bend in the stairlift rail, the speed of the carriage must be reduced. For the purposes of this disclosure a negative transition bend is defined as a bend in a vertical plane that reduces in inclination when traveling in an upward direction. The speed reduction is necessary to release sufficient battery power to operate the chair leveling mechanism and/or to ensure that the speed reference point (as defined in EN 81-40:2008 (E)) on the carriage, which is some distance above the axis of rotation of the bend, does not exceed a prescribed speed. A problem thus arising is that, if there is a failure in the main drive while the carriage is passing through a negative transition bend, the OSDD will also be operating at a lower speed and the carriage will therefore move a significant distance before the OSDD is not just brought back up to the rated speed, but beyond the rated speed to the tripping speed.
It is an object of the present invention to provide a stairlift, and/or an over-speed detection device and/or over-speed governor for a stairlift, and/or a method of configuring and/or operating a stairlift that will go at least some way to addressing the aforementioned drawbacks and/or requirements; or which will at least provide a novel and useful choice.